Present day refrigeration, air conditioning, and heat pump systems are generally quite simple in nature, especially in the smaller sizes, and deal with a very simple refrigeration cycle. In this relatively conventional, simple cycle, high pressure refrigerant vapor is condensed in a system condenser resulting in a relatively warm liquid refrigerant. This relatively warm liquid refrigerant is then ducted to an expansion device wherein the pressure is reduced to the evaporating level of the system. In the process of this pressure reduction, the sensible energy present in the liquid is used to evaporate a portion of this same liquid immediately prior to entering the evaporator. The liquid leaving the condenser is normally close to what is called the saturated liquid state. As the pressure is reduced on this saturated liquid, boiling will commence. In order to sustain the boiling process, energy is required. The energy comes from the liquid itself during the normal expansion process. In a typical cycle, a considerable amount of refrigerant vapor is generated during this normal expansion process. Since this refrigerant vapor has already evaporated, it is obvious that it can pick up no more energy through additional evaporation. Only the remaining liquid (after expansion) can be evaporated, thus picking up energy from a source which in turn is cooled. The vapor generated during the expansion process must be inducted into the compressor and compressed back up to the condensing level in order to repeat the cycle. It is obvious that this vapor, so generated, must require a certain portion of the compressor displacement and thus, it prevents that portion of the compressor displacement from taking in vapor that was indeed generated by the source evaporating the remaining liquid after expansion. While this particularly wasteful process has not caused too much concern, especially in the smaller sized air conditioning cycles, it has been most wasteful in the higher compression ratio refrigeration cycles. It is one of the primary reasons for very serious capacity deterioration in air source heat pumps when operating at the lower ambients, and it has been recognized and dealt with in the larger two stage air conditioning cycles.
In the larger two stage refrigeration systems, a device known as a flash gas economizer is commonly used. In this system, the warm condensed liquid is reduced in pressure to that level corresponding with the inlet pressure of the second stage compressor. In the process of doing this, a significant portion of the energy present in the warm condensed liquid is, therefore, removed prior to this liquid undergoing the final stage of expansion immediately preceding the evaporator. This has made an improvement in the performance of these larger air conditioning systems generally ranging between 5 to 10%, depending upon the compression ratio or lifts encountered. In other words, the increase in evaporator capacity is greater than the increase in system power requirements with the performance being defined as the ratio of evaporator energy divided by compression energy requirement.